DRIFT Study on Cerium−Tungsten/Titiania Catalyst for Selective Catalytic Reduction of NOx with NH3

CeO2/TiO2 and CeO2−WO3/TiO2 catalysts prepared by impregnation method assisted with ultrasonic energy were investigated on the selective catalytic reduction (SCR) of NOx (NO and NO2) by NH3. The catalytic activity of 10% CeO2/TiO2 (CeTi) was greatly enhanced by the addition of 6% WO3 in the broad temperature range of 200−500 °C, the promotion mechanism was proposed on basis of the results of in situ diffuse reflectance infrared transform spectroscopy (DRIFT). When NH3 was introduced into both catalysts preadsorbed with NO + O2, SCR would not proceed except for the reaction between NO2 and ammonia. For CeO2/TiO2 catalysts, coordinated NH3 linked to Lewis acid sites were the main adsorbed ammonia species. When NO + O2 was introduced, all the ammonia species consumed rapidly, indicating that these species could react with NOx effectively. Two different reaction routes, L-H mechanism at low temperature (<200 °C) and E-R mechanism at high temperatures (>200 °C), were presented for SCR reaction over CeO2/TiO2 catalyst. For CeO2−WO3/TiO2 catalysts, the Lewis acid sites on Ce4+ state could be converted to Brønsted acid sites due to the unsaturated coordination of Cen+ and Wn+ ions. When NO + O2 was introduced, the reaction proceeded more quickly than that on CeO2/TiO2. The reaction route mainly followed E-R mechanism in the temperature range investigated (150−350 °C) over CeO2−WO3/TiO2 catalysts. Tungstation was beneficial for the formation of Ce3+, which would influence the active sites of the catalyst and further change the mechanisms of SCR reaction. In this way, the cooperation of tungstation and the presence of Ce3+ state resulted in the better activity of CeO2−WO3/TiO2 compared to that of CeO2/TiO2.